A magnetic disc unit using a magnetic disc is conventionally known. The magnetic disc, which may be a flexible disc or a hard disc, is loaded into the unit for recording or reproducing information signals on a uniform magnetic layer which is formed on the magnetic disc. Recording and reproduction are carried out with a magnetic head forming part of the magnetic disc unit. The magnetic head is positioned by a sector servo mode to carry out tracking adjustment.
FIG. 13A illustrates the sector servo mode with respect to a magnetic disc 101 controlled in rotation by the so-called CAV (constant angular velocity) mode. Each sector region as a unit for data recording is spatially divided into a servo region Zs and a data region Zd. The servo region Zs is usually arranged at the top of the sector region.
The servo region Zs has a servo pattern Ps arranged in a zigzag pattern with respect to the center of the track as shown in FIG. 13B. In this example, two servo patterns Ps1 and Ps2 are arranged in the zigzag pattern in each sector. Each of the servo patterns Ps1 and Ps2 has a larger width Ts (width always being defined as the direction of diameter of the disc) than the width Tw of a recording track pattern Pw in the data region Zd. Thus, one servo pattern is used for two adjacent tracks. An expanded diagram of each of the servo patterns Ps1 and Ps2 is shown in FIG. 13B. A magnetic domain having both magnetization directions is used as a unit. The magnetic domains are magnetized in the direction of the track. The magnetic readout width Wr of a magnetic head H is set to be substantially the same as the width Tw of the recording track pattern Pw. Accordingly, the magnetic readout width Wr is smaller than the width Ts of the servo patterns Ps1 and Ps2.
Next, positioning control of the magnetic head H based on the two servo patterns Ps1 and Ps2 in one sector is explained. When the magnetic head H traces the third track from the left and is positioned slightly toward the left side of the center of the third track as shown in FIG. 13B, an output level S1 of a reproduction signal passing the first servo pattern Ps1 is smaller than an output level S2 of the reproduction signal passing the second servo pattern Ps2. The relative output levels are shown in FIG. 14. This phenomenon is caused by the fact that a transit area of a gap in the magnetic head H is larger in the second servo pattern Ps2. By comparing the output levels S1 and S2 of the reproduction signal, how far the position of the magnetic head is shifted from the center of the track can be determined.
Specifically, the first reproduction signal S1 is produced when the head passes by the first servo pattern Ps1. This signal S1 is delayed for a predetermined period by a delay circuit. The delayed version of signal S1 is compared with the second reproduction signal S2 which is produced when the head passes by the second servo pattern Ps2. The two signals are compared by a differential amplifier. The output of the differential amplifier produces a tracking error signal St which can be used to determine the position of the magnetic head H. The tracking error signal St is supplied to a servo circuit for controlling the magnetic head H. A tracking actuator is connected to the servo circuit and is driven by the servo circuit. Thus, the position of the center of the magnetic head H is adjusted to follow the center of the track.
The conventional magnetic disc 101 is provided with a guard band between tracks for avoiding crosstalk from the adjacent track. Consequently, the track pitch Tp of the magnetic disc is larger than the width of the magnetic head H (the magnetic readout width Wr or the magnetic writing width Ww) in the conventional magnetic disc unit. In general, the ratio Wr/Tp of the magnetic readout width Wr and the track pitch Tp, and the ratio Ww/Tp of the magnetic writing width Ww and the track pitch Tp is not more than about 0.8 for securing good S/N in the conventional magnetic disc unit.
The output characteristics of the tracking error signal St are explained by reference to FIG. 15 in the case of the magnetic head H tracing an arbitrary position along the center of the track. The axis of ordinate represents the output level of the tracking error signal St from the differential amplifier and the axis of abscissas represents the position of the magnetic head H along the direction of diameter of the magnetic disc. As seen from the graph, when the magnetic head H passes the center of the track Tc, the output level of the error signal St is 0. The output level of the error signal St shifts to a positive or negative level as the magnetic head H shifts from the center of the track Tc. However the ratio Wr/Tp and the ratio Ww/Tp are not more than about 0.8, and Wr is set to be smaller than Ts in the conventional case. Therefore the magnetic head H happens to trace within either servo pattern Ps1 or Ps2. The error signal St is substantially constant when the magnetic head traces within either the pattern Ps1 or Ps2. That is, there is a dead zone in which the level of the tracking error signal St does not change despite the change of the position of the magnetic head H in the conventional case. This phenomenon also occurs when the track pitch Tp and the width Ts of the servo pattern Ps1 or Ps2 are substantially the same, as shown in FIG. 16. Thus, a dead zone in the signal is generated, indicating that the error signal St is substantially constant between when one end (the left end in the drawing) of the magnetic head H is positioned on one end (the left end in the drawing) of the first sample servo Ps1, and a state in which the other end (the right end in the drawing) of the magnetic head H is positioned on the other end (the right end in the drawing) of the sample servo Ps1.
Accordingly, the position of the magnetic head H cannot be correctly determined when the magnetic head traces on the dead zone. This causes the problem that the magnetic head H doesn't move to the correct position (in the direction of diameter) when the magnetic head either reproduces data of a demanded address (sector) or writes data in the address (sector). That is, step jump operation, track jump operation and seek operation cannot be carried out accurately. This causes long delays in accessing data.
When the magnetic readout width Wr is substantially the same as the width Ts of the servo pattern and is smaller than the track pitch Tp as shown in FIG. 17A, the change of the signal is small even though the dead zone is improved as shown in FIG. 17B. Therefore, when the magnetic head passes the dead zone, the servo gain becomes extremely small and the magnetic head isn't positioned accurately by the servo circuit.
Thus, it is desirable to eliminate the dead zone in order to position the magnetic head H more accurately and speedily. In addition, since the relation between the magnetic readout width Wr, the magnetic writing width Ww and the width Tw of the recording track pattern Pw is approximately Wr=Ww=Tw in the conventional magnetic disc unit, when the magnetic head is off the track from the recording track pattern Pw even slightly at the time of reproduction, the output is lower than if the head is on the track as shown in FIG. 18A. If the magnetic head is off the track at the time of recording, a portion of the previously recorded information remains. This remaining information acts as noise at the time of reproduction. Thus, the overwrite S/N (signal to noise ratio) of the magnetic head is degraded. When the magnetic disc unit is vibrated, the problem of off tracking is generated more frequently. This limits the circumstances under which the magnetic disc unit can be used.